Microwave radiation is becoming of increasing importance as a very efficient and effective tool for the heat treatment and sintering of materials such as refractory metal oxides. Microwave radiation very efficiently and rapidly heats oxide materials to the relatively high temperatures required for sintering. Also, microwave radiation penetrates the particulate mass of oxide material to provide a more uniform sintering of the oxide material than heretofore realized with the relatively non-uniform beat up provided by conventional furnaces. However, in spite of many advantages, it has been found that there are some problems attendant with the use of microwave radiation for sintering of many oxide materials due to their being essentially transparent or noncoupling with microwave radiation even at relatively high frequencies of up to about 2.45 GHz at temperatures in a range of about room temperature up to about 900.degree. C.
In order to effect the sintering of such oxide materials with microwave radiation, materials capable of absorbing or coupling with microwave radiation such as boron carbide, silicon boride, titanium nitride, or uranium oxide were previously admixed with the particulate oxide material to permit the sintering of the oxide material with microwave radiation. In these instances the additive couples to the microwave radiation, transferring heat to the surrounding particulates of oxide material by contact and/or convection. This heating continues until the oxide material reaches a temperature of several hundred degrees centigrade where high frequency relaxation mechanisms such as ionic conduction and molecular vibrations occur in the oxide material. These relaxation mechanisms cause the oxide material to couple or absorb microwave radiation so that the oxide material can be heated thereby to the desired sintering temperature.
While the addition of coupling material to the body of oxide particulates forming a compact to be sintered has proven to be useful for the microwave heating of oxide materials from room temperature up to the temperature where the oxide materials couple with the microwave radiation, the purity of the sintered oxide materials may be compromised by the presence of the added coupling material. Further, the properties and characteristics of the sintered oxide material with this added impurity are often altered sometimes deleteriously.